1. Field of the Invention
The present invention relates to data communications, and more particularly, to digital impairment learning for pulse-code modulation (PCM) modems used in transmission paths compatible with digital modulation.
2. Description of the Related Art
Much of the public switched telecommunications network (PSTN) is implemented using digital data transport. Nonetheless, significant portions of the PSTN are still based on analog technology. For example, the “local loop” portion of PSTN that connects a telephone subscriber to a central office (CO) is typically an analog loop. Additionally, analog portions may exist at other points along a communications path, e.g., as an analog channel in an otherwise digital circuit.
To achieve high downstream data rates in the PSTN, the current generation of 56 Kbps modems no longer treat the communications path as an analog channel. Instead, such modems assume that there is only one (1) analog portion in a downstream transmission path from a digitally connected server modem to a client modem connected to an analog local loop. This configuration is reasonable in areas where most Internet Service Providers (ISPs) and business customers are digitally connected to the network and allows data signaling rates of up to 56 Kbps in the downstream transmission path.
Although a variety of similar designs are available, modems conforming to the ITU-T Recommendation V.90 are illustrative. See generally, ITU-T Recommendation V.90, A Digital Modem and Analogue Modem Pair for Use on the Public Switched Telephone Network (PSTN) at Data Signalling Rates of up to 56 000 Bit/S Downstream and up to 33 600 Bit/S Upstream (09/98) hereinafter referred to as “Recommendation V.90”), the entirety of which is incorporated by reference herein. Recommendation V.90 defines a method for signaling between a modem connected to an analog loop (the analog modem) and a modem connected to the digital trunk (the digital modem). Modems in accordance with Recommendation V.90 take advantage of this particular arrangement to increase the data rate from the digital modem to the analog modem.
Earlier generations of modems, e.g., those conforming to ITU-T Recommendation V.34 and/or earlier standards, suffered from quantization noise introduced in the conversion of analog signals to digital, Pulse Code Modulation (PCM) codewords for transmission in digital portions of the PSTN. Modem modems, including modems conforming to Recommendation V.90, avoid the effects of PCM quantization by using a form of amplitude modulation that allows voltage amplitude levels to be chosen from the quantization levels of the PCM coder/decoder (codec) in the CO. Recommendation V.90 matches the quantization levels to potentially eliminate quantization noise entirely in a downstream path from a server modem to a line interface. Thus, Recommendation V.90 achieves higher data rates when there are no analog-to-digital conversions between a V.90 digital modem and the PSTN. A digital V.90 modem transmits PCM codewords that are converted to discrete analog voltage levels in the local CO and are sent to the analog V.90 modem via the analog local loop. The analog modem's receiver then reconstructs the discrete network PCM codewords from the analog signals received.
There is no specific technique provided in Recommendation V.90 by which the analog client modem is to decide whether PCM signaling can be supported by the downstream channel. Rather, the client modem must make an inference about the condition of the channel during the training process. Moreover, part of the modem training procedure, defined in Recommendation V.90, includes estimating the digital impairments present on the telephone channel. Several common sources of digital impairments are Robbed Bit Signaling (RBS), transcoding (e.g., A-law to μ-law conversion), and digital attenuation pad.
Using current techniques, 56 Kbps downstream signaling rates can often be achieved. However, actual signaling rates may be limited by distortion introduced in the digital backbone itself. One source of distortion is Robbed Bit Signaling (RBS). RBS is an in-band signaling technique used in some portions of the PSTN to perform control functions such as conveyance of ring and call progress indications in the telephone network. In short, RBS involves modification by the PSTN of data transmitted. Generally, a least significant bit (LSB) of certain PCM codewords may be used (or usurped) by a portion of the digital backbone. Although usurpation of bits is generally acceptable when codewords carry a voice signal, RBS effectively acts as noise or distortion when codewords carry a data signal. Moreover, RBS limits the information carrying capacity of a communications channel that includes a portion employing it.
Recognizing these limitations, techniques have been developed for detecting, characterizing and mitigating RBS. For example, U.S. Pat. No. 5,875,229 to Eyuboglu et al., proposes detection and characterization of RBS during a “training” phase prior to other training operations such as initialization of equalizer coefficients. Eyuboglu's characterization technique is based on counting LSB values equal to logic zero and logic one in each of 24 intervals of a received training signal. U.S. Pat. No. 5,859,872 to Townshend also proposes a scheme for handling RBS in which RBS is detected during an initial training phase. In Townshend, a decoder first attempts to equalize a received training signal having a known pattern by minimizing the difference between its output and a known pattern under the assumption that no robbing of bits occurred. The decoder then measures the average equalized values at each of six phases and determines for each phase which of 4 bit robbing schemes (including no bit robbing) has been employed. Once the bit robbing that occurred in each phase is determined, the equalization process is rerun, since the first equalization was performed without knowledge of the bit robbing.
Recommendation V.90 specifies a protocol between digital and analog PCM modems, whereby a Digital Impairment Learning (DIL) sequence is supplied to allow the analog PCM modem to detect digital impairments in a communication path. In this way, the analog PCM modem may define a constellation (or constellations) that tend to avoid or compensate for the discovered impairments. The protocol defined in Recommendation V.90 allows each manufacturer to define a Digital Impairment Learning (DIL) sequence to meet its objectives.